The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Known electric motors include permanent magnet electric motors that transform electric power to mechanical torque. Permanent magnet electric motors may be multiphase interior permanent magnet (IPM) electric motors that include an annular stator into which a rotor element is inserted. Known stators include an annular stator core and a plurality of electrical windings. Known stator cores include a plurality of radial inwardly projecting tooth elements that are parallel to a longitudinal axis of the electric motor and define an inner circumference of the stator. Contiguous radial inwardly projecting tooth elements form radially-oriented longitudinal slots. Electrical windings are fabricated from strands of suitable conductive material, e.g., copper or aluminum, and are woven or otherwise arranged into coil groups that are inserted into the radially-oriented slots between the tooth elements. Electrical windings are arranged electrically in series in circular fashion around the circumference of the stator core, with each electrical winding associated with a single phase of the electric motor. Each coil group of the electrical windings provides a single pole of a single phase of motor operation. The quantity of radially-oriented slots in the stator core is determined based upon the quantity of phases and poles of the electrical wiring windings for the electric motor. Thus, a three phase, two-pole induction motor will have electrical windings that are configured as six coil groups, with the coil groups configured in six slots or a quantity of slots that is a multiple of six. Current flow through the electrical windings is used to generate rotating magnetic fields that act on a rotor to induce torque on a shaft of the rotor.
Known rotors for permanent magnet electrical motors include a rotor core attached to a rotating shaft that defines an axis of rotation. Known rotors have a plurality of rotor magnets positioned around the circumference near an outer surface of the rotor core, with each rotor magnet aligned longitudinally with the axis of rotation.
Known permanent magnet electrical motors include an air gap between tooth elements of a stator and an outer surface of a rotor. An air gap is a design feature that is necessary to accommodate manufacturing tolerances, facilitate assembly, and address other known factors. An air gap is preferably minimized, as an increased air gap correlates to reduced magnetic flux and thus reduced output torque.
When electric current flows through the stator windings, a magnetic field is induced along the electrical windings associated with a single phase of the section of the stator that acts upon the rotor magnets of the rotor element. The magnetic field induces torque on the rotating shaft of the rotor. When the magnetic field induces sufficient torque to overcome bearing friction and any induced torque load on the shaft, the rotor rotates the shaft.
In operation, discontinuities in motor torque output including torque ripples are associated with magnitude of an air gap. The air gap and the associated discontinuities in the motor torque output affect maximum motor torque output and affect noise, vibration, and harshness performance of the electric motor.
Design of permanent magnet electric motors includes factors related to magnetic, mechanics, thermodynamics, electronics, acoustics, and material sciences. It is known that performance requirements, packaging constraints and costs impose constraints on motor design that affect design features. Known performance requirements include maximum motor torque output, torque ripple, and cogging torque, which affect noise, vibration, and harshness performance of the electric motor. Known permanent magnet electric motors have flux distribution due to the permanent magnets and the armature magneto-motive forces that is non-sinusoidal with respect to the angular rotor position. The non-sinusoidal flux distribution introduces torque pulsations that are reflected as speed ripple, noise and vibration. Torque pulsations may degrade performance of permanent magnet electric motors and are preferably minimized. Torque pulsations affect performance, including efficiency, audible noise, vibration, and harshness. Effects upon performance vary at different operating points, i.e., torque pulsations may vary in response to operating at different speeds and torque outputs. Known strategies to reduce or minimize torque pulsations include skewing locations of magnets in a rotor to minimize torque ripple, adjusting specific design features of a stator and/or a rotor to achieve a minimum torque ripple or achieve a maximum motor torque output at a specific operating point, and executing control strategies to generate an inverse torque component through the stator current.